The order of the registers in

Hi all,

Can anyone explain how the order of the registers in is determined?

E.g. for RISCV:

namespace RISCV {

enum {


X0 = 1,

X1 = 2,

// (…)

F31_F = 96,




How does this relate to the content of the file? It seems like the enum is generated with the same ordering as the register records appear when running llvm-tblgen.

I have shuffled with my definitions but don’t seem to be able to influence this order.

Root issue:

My architecture has specific registers for constants, a separate register class. I run a pre-regalloc allocator to allocate constants to registers from that class (because they need to be persistent and not be reused etc).

I can simply ‘MO.setReg(constRegId++)’ in my allocator but then I would need to know the number of the register, which apparently is not the same as the one defined in my file:

foreach regid = 447-510 in {

def C#regid : XXXConstReg<regid, “c”#regid, [“c”#regid]>, DwarfRegNum<[regid]>;


If there is a way to get the register range for a certain register class such that I can allocate machine operands to a physical register in that class, then that would solve my issue.

Mvg Simon de Vegt

I believe the sorting is handled by LessRecordRegister in include/llvm/TableGen/Record.h

Register names are broken down into parts that contain either digits or non-digits. Registers with fewest parts come first. Then it is sorted by the non-digit parts. If those are all equal, it will be sorted by the digit parts.